CIE A Level Biology (9700) 2019-2021

Revision Notes

8.1.7 Haemoglobin & Carbon Dioxide

Haemoglobin & Carbon Dioxide

  • Haemoglobin is a large globular protein composed of four polypeptide chains, each chain containing an iron group
  • Each haemoglobin molecule can pick up four oxygen molecules
  • This is a reversible reaction and forms oxyhaemoglobin:

4O2 + Hb (Haemoglobin)  → HbO8 (Oxyhaemoglobin)

  • The binding of the first oxygen molecule alters the formation of the molecules and so the other three oxygen molecules are able to bind much faster than the first
  • The reverse of this process happens when oxygen dissociates at the tissues and the last oxygen molecule is the hardest to remove

Carbon dioxide

  • Waste carbon dioxide diffuses from tissues and into the blood following aerobic respiration
  • There are three main ways in which carbon dioxide is transported around the body
  • A very small percentage of carbon dioxide (~ 10 %) dissolves in blood plasma, forming H2CO3
  • A much larger percentage (~ 70 %) of carbon dioxide dissolves in the cytoplasm of red blood cells
  • Red blood cells contain the enzyme carbonic anhydrase which catalyses the reaction between carbon dioxide and water
    • Without carbonic anhydrase this reaction proceeds very slowly. The plasma contains very little carbon anhydrase hence H2CO3 forms much more slowly in plasma than in the cytoplasm of red blood cells
  • Carbonic acid dissociates readily into H+ and HCO3- ions :

CO2 + H2O ⇌ H2CO3 ⇌ HCO3 + H+

  • The increase in H+ concentration results in a decrease in blood pH, which alters the structure of haemoglobin, encouraging the dissociation of oxyhaemoglobin to release oxygen
    • This is beneficial – when levels of carbon dioxide are higher, rates of aerobic respiration are greater and therefore the need for oxygen is higher
  • Hydrogen ions (protons) can combine with haemoglobin, forming haemoglobinic acid
  • Carbon dioxide can also bind to amino acids and therefore haemoglobin, forming carbaminohaemoglobin – this accounts for ~ 20 % of carbon dioxide transport in the blood

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